Treatment for a diseased or damaged disc can involve the removal of the natural, damaged disc tissue, and the subsequent replacement of the disc with an implant, such as an interbody cage or fusion device, or a disc prosthesis. The implant should have an appropriate size and shape to complement the normal height of the disc and to contour the vertebral endplates adjacent the disc space to provide stability and, if a fusion device is being implanted, to promote fusion. If the shape of the vertebral endplates does not match the shape of the implant, shifting can occur resulting in misalignment of the vertebrae. Accordingly, it is important for the implant to correspond as closely as possible to the region of the intradiscal space that is receiving the implant.
Selection of a properly sized implant can be difficult due to the enclosed nature of the intervertebral disc space. X-rays generally reveal very little about the particular size and shape of the intradiscal space, and thus surgeons often have to rely on an estimated shape and size based on physiological factors, such as the patient's height, weight, etc., as well as the position of the vertebrae. While this method can be sufficient, the selection of an improperly sized implant can lead to problems. An oversized implant, for example, will be difficult to position between the adjacent vertebrae and can lead to long term problems once implanted. Moreover, due to the enclosed nature of the intervertebral disc space, it is virtually impossible for a surgeon to accurately evaluate the size and shape of the cavity, much less the matching of the implant with the vertebral endplates. Unfortunately, the sizing problem is not always discovered until the surgeon attempts to position the implant between the vertebrae. As a result, the implant will have been in direct contact with bodily fluids and will be contaminated and not recommended for reinsertion/implantations. Some implants can be virtually impossible to resterilize due to the nature of the materials from which they are made. In such cases, it is necessary to discard an expensive device.
To overcome this problem, trial implants and trial implant kits have been developed to assist surgeons in selecting an implant having the appropriate size and shape. U.S. Pat. No. 6,113,639 of Ray et al. discloses, for example, a trial implant kit containing several trial implants, each sized and shaped to simulate the size and shape of an available prosthetic implant. The surgeon can select an implant from the kit to temporarily position within the disc space to evaluate the size of the intradiscal space and the fit of a sample prosthesis. A contrast material can be injected into the nucleus cavity to view the trial implant with respect to the intradiscal space via a fluoroscope.
While fluoroscopy or x-ray can be effective to verify the placement of a trial implant, the image produced can be distorted by the large, opaque implant. This distortion can either shield or completely obscure the anatomical matching that the surgeon desires to verify. It can also be difficult to accurately assess whether the implant is in close contact with the complex geometries of the adjacent vertebral endplates.
Prior to implanting such an implant, it is also necessary to adequately prepare the surface of the vertebral endplates to ensure the best possible fit of the implant. This procedure typically entails removing the damaged disc and cleaning and shaping the endplate of each of the vertebrae. The sufficiency of the preparation of the endplate surface can influence the eventual fit of the implant and thus the outcome of the surgical procedure. However, existing techniques for assessing the endplate preparation to receive a desired implant are often insufficient to determine whether the endplate surfaces have been properly cleaned and the intervertebral space has been adequately prepared to accept the implant. In particular, it is difficult or impossible to view the disc space and the endplate surfaces. It is also challenging to accurately make free volume determinations or determine endplate contact area locations.
Accordingly, there remains a need for methods and devices for evaluating preparation of the surfaces of the adjacent vertebral bodies for accepting an implant and/or for performing a spinal fusion procedure.